What it means
The alternative fuel hydraulic pump pressure sensor circuit signal is higher than expected.
At a glance
- Usually lights the check-engine light
- Affects emissions; the car can fail an emissions test
- The engine or gearbox may go into limp mode
How it works
The alternative-fuel hydraulic pump pressure sensor reports hydraulic pressure to the control module. The module monitors the sensor signal and sets P2A78 when it reads higher than expected, which can result from a signal wire shorted to battery voltage, an open sensor, or poor connector contact.
Common causes
Most likely first.
- Signal wire shorted to B+
- Open pressure sensor
- Connector pins corroded - high contact resistance
Symptoms
- Alternative-fuel system warning light, if equipped
- Reduced power or poor acceleration
- Hard starting or stalling
How to diagnose it
- Use a scan tool to save freeze-frame data and compare the reported hydraulic pressure with operating conditions to see when the high reading occurred.
- Inspect the hydraulic pump pressure sensor connector and wiring for corrosion, damage, loose pins, or a signal wire touching a B+ source.
- With the wiring diagram, check the sensor supply, ground, and signal circuits for continuity and shorts to battery voltage.
- Compare the sensor signal and scan-tool pressure reading with the manufacturer's specification under the conditions in the freeze frame.
- If the wiring and connector pass, test the pressure sensor for an open circuit and replace it if it fails the manufacturer's checks.
Common fixes
- Clean and securely reconnect corroded or loose sensor connector terminals.
- Repair a signal wire shorted to B+ or damaged wiring at the pump pressure sensor.
- Replace the open alternative-fuel hydraulic pump pressure sensor if circuit tests confirm it has failed.
Can I keep driving?
Avoid hard acceleration and long trips because the alternative-fuel system may limit power or cause hard starting or stalling. If it stalls or power drops sharply, stop somewhere safe and arrange repair before continuing.
A code shows where the control module detected a fault, not always which part failed. Check the causes in order and confirm with a test before replacing parts. Have several codes? Check them together
This is a generic code: it means the same on every make. The car’s maker may add more detail in its own service information.